LOL, thanks.  Not what I wanted to hear after days of debugging!  Damn and I searched for known issues and didn't find anything. 

I agree with your comment about disabling by default.  Or completely removing it as if only used by dial up modems then what is the point...

Regards

Ashley

Context: Embedded project, STM32H7, GCC Arm V9, using PPP to communicate with a ublox Nina W152 WiFi module.  Lwip V2.1.2.

We have the need to build our code size optimised (-Os).  We use a debug build (Og) and a release build with Os.  We have had very robust and reliable communications on debug build.  Unfortunately since building a release build, the PPP connection to the module seemed unstable and erratic and communications was unreliable.

After several days of debugging it was traced down to a single function vj_uncompress_tcp in vj.c.  By disabling size optimisation of just this function (leaving it at Og). communications is excellent.  Compiling this function with Os breaks communication.  This is completely repeatable.

I have not delved into exactly what happens inside this function causing it to become an issue.

To fix the problem (when the entire project is built with Os):

#pragma GCC push_options
#pragma GCC optimize ("Og")
int
vj_uncompress_tcp(struct pbuf **nb, struct vjcompress *comp)
{
  ...
}
#pragma GCC pop_options

For now we have disabled VJ_SUPPORT in lwipopts.h as was not really required for this use case anyway.